http://shockwave.com
I don't know why...but I find this game so addicting.I used to play with my friends during computer class and my other friends when I would stay up all night.
Its probably my favriote free online game I have ever played.That or Gunbound...

The idea is,pretty much,you draw a picture and the other people try to guess it.Then other people draw and you try to guess it...I thought it was stupid at first and the only reason I went to it was to annoy my friends...but my god did we all love it XD